Fix warnings for unknown keys

When starting a new game I got the following warnings printed in the terminal:
ignoring unknown key  set to  in file ./mods/fantasycore/powers/powers.txt
ignoring unknown key range set to 0 in file ./mods/fantasycore/powers/powers.txt

The first is due to the (presumably) accidental 'F' in front of the comment/headline. The second one was fixed by changing Thunderstrike's range to target_range.